Metal casting processes with vacuum and pressure

ABSTRACT

A gas permeable mold, preferably an investment mold, is cast under a vacuum. The pressure on the molten metal exposed through the opening of the mold is raised to atmospheric pressure after casting, and a vacuum is applied to the outside of the mold during solidification of the metal.

United States Patent Watts [451 Dec. 12, 1972 [54] METAL CASTINGPROCESSES WITH VACUUM AND PRESSURE [72] Inventor: Claude 11. Watts,Mayfield Heights,

Ohio

[73] Assignee: Precision Metalsmiths,lnc.

[22] Filed: Dec. 9, 1970 [21 Appl. No.: 96,517

[52] US. Cl. ..164/62, 164/65, 164/255,

164/256 [51] Int. Cl ..B22d 27/14, 822d 27/15 [58] Field ofSearch.....l64/62, 65, 61, 255, 256, 258

[56] References Cited UNITED STATES PATENTS 1,070,500 8/1913 Mackenzie.164/258 2,133,634 10/1938 Rohn ......l64/258 3,336,970 8/1967 Watts etal. ..l64/62 3,620,296 11/1971 Barbieri et al ..l64/62 X I PrimaryExaminer-Robert D. Baldwin Attorney-Watts, Hoffmann, Fisher & l-leinke[57 ABSTRACT A gas permeable mold, preferably an investment mold, iscast under a vacuum. The pressure on the molten metal exposed throughthe opening of the mold is raised to atmospheric pressure after casting,and a vacuum is applied to the outside of the mold during solidificationof the metal.

12 Claims, 1 Drawing Figure PATENTEnuu: 12 I972 r 3,705,615

INVENTOR. CLAUDE H. WATTS BY mm I ATTOENEKS.

METAL CASTING PROCESSES WITH VACUUM AND PRESSURE BACKGROUND OF THEINVENTION The present invention relates generally to metal castingprocesses, and more specifically to methods of casting investment molds,including both shell molds and bulk investment molds.

The invention is particularly concerned with providing methods forproducing sound investment castings made of aluminum alloys and othermetals which are subject to similar defects. Investment cast aluminumalloys are recognized to be especially susceptible to air entrapment andporosity, and in the past these conditions have been a major cause ofrejected castings. In some instances, for example, when casting Type 355and Type 356 aluminum alloys, avery high rejection rate of from 75percent'to 90 percent has been experienced, Another problem encounteredin the investment casting of metals such as aluminum alloys is that ofthe metal retracting from the surfaces of the investment mold duringsolidification. This condition is frequently referred to as draw-back.

SUMMARY OF THE INVENTION According to the present invention, there isprovided a method of casting a gas permeable mold, such as an investmentmold, comprising the steps of introducing molten metal while under avacuum into an opening of the mold to fill a cavity, thereafterincreasing the pressure on the molten metal through the mold opening,and applying a vacuum to the outside of the mold during solidificationof the metal.

According to a more preferred aspect of the present invention, there isprovided a method of casting a gas permeable mold, such as an investmentmold, which comprises the steps of placing the mold in a mold chamber sothat the opening of the mold communicates with a vacuum casting chamberwhich is sealed from the mold chamber, evacuating the vacuum castingchamber and then introducing molten metal into the opening'of the moldto fill a cavity, thereafter restoring the casting chamber toatmospheric pressure while the metal in the mold is still molten, andapplying a vacuum to the outside of the mold by evacuating the moldchamber prior to solidification of the metal, and more preferably priorto introducing the metal into the mold.

The new methods provided by the invention can be used to cast manydifferent kinds of metals, and may be found particularly advantageous inproducing castings having thin wall sections. The new methods areespecially suited for casting aluminum alloys and the like which aresubject to air entrapment, porosity and drawback.

Other advantages and a fuller understanding of the invention will be hadfrom the following detailed description and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a diagrammatical,vertical cross-sectional view showing an apparatus for carrying out themethod of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrated apparatus is theinvention of Timothy L. Coghill and was developed after the date of thepresent invention. The apparatus is disclosed as the best known mode ofpracticing the invention, but no claim of inventorship is made thereto.

The illustrated apparatus comprises an enclosure 10 which is adapted todefine a mold chamber 11 and a vacuum casting chamber 12. The moldchamber 11 is connected through a conduit 13 and a valve (not shown) toa suitable vacuum source, such as a vacuum pump or line. The vacuumcasting chamber.12 also is connected to a suitable vacuum source througha conduit 14 and a valve (not shown). The valves in the conduits l3, 14are three-way valves which can be actuated to connect the chambers 11,12 to vacuum source and to the atmosphere.

A platform 20 is provided within the chamber 11 for supporting a mold.The platform 20 may be comprised of a metal plate 21 on which isdisposed a gasket 22. Suitable gaskets may be made from asbestos pads,insulation or blankets formed of refractory fibers, such asaluminasilica fibers, and the like. A hole 23 is formed through theplate 21 and the gasket 22, and the plate 21 is supported above thebottom of the chamber 11 by suitable supports 24. The height of thesupports 24 is preferably adjustable so that the top of the moldsupported on the platform can be positioned in the desired plane.

As shown, the enclosure 10 includes a lower section 30 which defines themold chamber 11 and an upper section '31 which defines the vacuumcasting chamber 12. A suitable sealing gasket 32 is provided between thetwo sections of the enclosure. The upper section 31 can be separatedfrom the lower section 30 to permit a mold to be placed into and removedfrom the mold chamber 11. The upper section 311 includes a window 33through which an operator can observe the casting operation.

A ladle 35 is pivotally mounted in he casting chamber 12 for use inpouring metal into a mold supported on the platform 20 in the moldchamber 11. A handle (not shown) which is used to tip the ladle extendsthrough a side of the enclosure section 31. The upper section 31 of theenclosure 10 is provided with a plate 36 which can be removed to fillthe ladle 35.

An illustrative investment mold which can be cast by the practice ofthis invention is generally designated by reference numeral 45. The mold45 is comprised of a refractory body 46 having an opening 47, a centralpassage 48, and a plurality of pattern cavities 49 which are gated intothe passage 48. As shown, a core 50 is positioned within the moldpassage 48. The core 50, which may be formed of refractory material, hasa closed end and an open end. A peripheral rim or step 51 is formed atthe lower, open end of the core. The rim or step 51 serves to center thecore 50 within the mold body 46 and substantially fills the passage 48so that little or no molten metal can squeeze past the core when themold is cast. A refractory pouring basin or collar 52 is shownpositioned on the top of the mold 45 around its opening 47.

The refractory mold body 46 may be made by conventional investmentmolding techniques. A plurality of patterns which are replicas of theparts to be cast in metal, including the necessary gates and risers, andwhich are formed of an expendable material, such as wax, are attached toa central sprue member to form what is known as a set-up or tree. Theset-up or tree is placed within a heat-resistant flask 53 which ispreferably perforated and the flask is filled with a cementitiousrefractory material known as the investment which hardens at roomtemperature around the set-up or tree. The investment material may be agypsum bonded refractory material which consists essentially ofcristobalite and fine silica powder together with accelerators andretarders to adjust the setting time. After hardening, the set-up isremoved from the mold body 46 to form the central passage 48 and themold cavities 49.

In carrying out the practice of this invention, the mold 45 is placed inthe chamber 11 on the platform gasket 22 and the core 50 is positionedwith its open end over the hole 23 of the platform. The mold chamber 11is then sealed from the vacuum casting chamber 12 with the opening 47 ofthe mold in communication with the vacuum casting chamber. This isaccomplished by a metal ring 60 which surrounds the upper end of themold and is supported by a shoulder 61 of the lower enclosure portion30. A rubber sealing ring 62 or the like is disposed between the ring 60and the shoulder 61. The seal is completed by a ring 63 which rests onthe upper end of the mold 45 and on the ring 60. The ring 63 may be madeof asbestos, rubber or other similar material.

When the mold 45 has been positioned in the manner described, the ladle35 is filled with molten metal and a vacuum is drawn in the castingchamber 12 to evacuate the interior of the mold. In accordance withconven tional practice, the metal is usually degased prior to beingplaced in the ladle, and therefore the metal can be introduced into themold 45 as soon as the desired vacuum has been obtained in the chamber12. The degree of vacuumin the chamber 12 can be varied depending uponsuch factors as the kind of metal being cast, the amount of metal, thesize of the mold, etc. A typical vacuum used for casting aluminum alloysis from 26 to 28 inches of mercury below atmospheric pressure.

After the molten metal has been poured into the mold 45, the pressure inthe casting chamber 12 is raised, preferably by relieving the vacuum andopening the chamber to atmospheric pressure. Atmospheric pressure on themolten metal exposed in the mold opening 47 acts to force the moltenmetal into the mold cavities to promote a complete fill and aids inpreventing draw-back as the metal subsequently solidifies.

A vacuum is drawn in the mold chamber 11 prior to solidification of themetal and preferably at the same time that the vacuum is drawn in thecasting chamber 12. The metal in the mold opening is maintained atatmospheric pressure and the vacuum applied to the outside of the moldis continued during at least partial solidification of the metal. Thedegree of vacuum in the chamber 11 which has been found sufficientsuccessfully to cast metals such as aluminum alloys is usually inches ormore of mercury below atmospheric pres sure. It is believed that theeffectiveness of the vacuum around the mold may be the result ofremoving air from the pores of the mold and thereby preventing any suchair from coming out of the pores prior to and during solidification.Whatever the explanation, it has been found that the application of thevacuum to the outside of the mold while the metal is under pressurethrough the mold opening results in the production of sound castingswhich are free of the defects heretofore encountered.

The solidification characteristics of the metal are affected by the rateof cooling and the temperature of the molds at the time of introducingthe metal. In order to promote a complete fill of the mold cavities, itmay be desired to cast the molds while they are hot. For example, whencasting Types 355 and 356 aluminum alloys, investment molds arefrequently at a temperature in the range of from about 300 to 800F., andpreferably at a temperature of approximately 400F. In other instances,for example, when producing castings having thick or heavy wallsections, itmay be desirable to pour the molds at room temperature inorder to chill the heavy wall sections as rapidly as possible.

'An example of the invention involved the casting of an investment moldsubstantially as shown in the drawing, the mold having been made by theconventional practice described above. A dome-shaped core was spacedapproximately one-half inch from the inside surface of the mold. Themold was cast at a temperature of 400F. with Type 356 aluminum alloywhich was poured at a temperature of 1,300F. The nominal composition ofthe alloy was 0.3 percent magnesium, 7 percent silicon and the balancealuminum. A vacuum of 26 inches of mercury below atmospheric pressurewas drawn in the casting chamber prior to introducing the metal into themold. Immediately after casting the mold, the casting chamber wasrestored to atmospheric pressure and a vacuum of from 15 to 20 inches ofmercury below atmospheric pressure was maintained in the mold chamber11. The metal was allowed to solidify for 2 to 3 minutes beforereleasing the vacuum in the mold chamber. The castings which wereproduced were found to be free of air entrapment and draw-back. Similarmolds were cast with Type 356 aluminum alloy following the sameprocedure except for the elimination of the step of applying a vacuum tothe mold body during solidification. The castings which were producedwere unsatisfactory because of extreme porosity and severe drawback.

As used herein, the term investment molds is meant to include bulk orsolid molds of the type shown in the drawing and ceramic shell molds. Itis to be understood that the methods provided by this invention are notlimited to the casting of investment molds of any specific type or whichare made of a particular refractory material, and that the mold 45 hasbeen disclosed only for the purpose of setting forth one specificexample of the new casting technique. The methods which have beendescribed may be used to advantage in casting bulk or solid typeinvestment molds which are not provided with cores such as shown at 50,ceramic shell molds, and other types of gas permeable molds, includingsand molds, etc.

Many modifications and variations of the invention will be apparent tothose skilled in the art in the light of the foregoing disclosure.Therefore, it should be understood that, within the scope of theappended claims,

the invention can be practiced otherwise than as specifically shown anddescribed.

What is claimed is: I

1. In the casting of a gas permeable mold having an opening into whichmolten metal is poured to fill a cavity of the mold, a method comprisingthe steps of:

a. evacuating the cavity of the mold and introducing molten metal whileunder a vacuum into the mold opening to fill the cavity,

b. thereafter increasing the pressure on the molten metal through themold opening, and

c. Applying a vacuum to the outside of the mold prior to solidificationof the metal.

2. In the casting of a gas permeable mold having an opening into whichmolten metal is introduced to fill a cavity of the mold, a methodcomprising the steps of:

a. applying a vacuum to the opening of the mold,

b. introducing molten metal under a vacuum into the mold opening to fillthe cavity,

c. thereafter relieving the vacuum applied to the mold opening so thatthe pressure on the molten metal in the mold is increased, and,

d. applying a vacuum to the outside of the mold prior to solidificationof the metal.

3. In the casting of a gas permeable mold having an opening into whichmolten metal is introduced to fill a cavity of the mold, a methodcomprising the steps of:

a. removing air form the inside of the mold through the mold opening,introducing molten metal under a vacuum into the mold opening to fillthe cavity,

c. thereafter exposing the opening of the mold to atmospheric pressurewhile the metal is still molten,

. applying a vacuum to the outside of the mold prior to solidificationof the metal, and

e. maintaining the mold opening exposed to atmospheric pressure andcontinuing to apply a vacuum to the outside of the mold as the metal atleast partially solidifies.

4. A method as claimed in claim 3 wherein the vacuum is applied to theoutside of the mold prior to the step of introducing molten metal intothe mold opening.

5. In the casting of a gas permeable mold having an opening into whichmolten metal is introduced to fill a cavity in the mold, a methodcomprising the steps of:

a. mounting the mold so that its opening is in communication with avacuum casting chamber,

. evacuating the chamber and then introducing molten metal into the moldopening to till the cavity,

c. increasing the pressure in the chamber while the metal in the mold isstill molten,

. applying a vacuum to the outside of the mold prior to solidificationof the metal, and,

e. continuing the application of the vacuum as the metal at leastpartially solidifies.

6. A method as claimed in claim 5 wherein the vacuum is applied to theoutside of the mold before introducing the molten metal into the moldopening.

7. In the casting of a gas permeable mold having an opening into whichmolten metal is introduced to fill a cavity of the mold, amethod'comprising the steps of:

a. mounting the mold in a mold chamber with the opening of the moldcommunicating with a vacuum casting chamber which is sealed from themold chamber, evacuating the vacuum casting chamber and then introducingmolten metal into the mold opening to fill the cavity,

c. thereafter restoring pressure to the vacuum casting chamber while themetal in the mold is still molten, and

d. evacuating the mold chamber prior to solidification of the metal.

8. A method as claimed in claim 7 wherein the mold chamber is evacuatedbefore the step of introducing the metal into the mold opening.

9. A method as claimed in claim 7 including the step of preheating themold before placing it in the mold chamber.

10. in casting a gas permeable mold having an opening into which moltenmetal is introduced to fill a cavity of the mold, a method comprisingthe steps of:

l a. evacuating the cavity of the mold and introducing molten metalunder vacuum into the mold opening to fill the cavity,

b. thereafter causing the pressure on the molten metal which is exposedthrough the opening of the mold to be increased,

c. providing-a sealed chamber around the mold,

. evacuating the chamber to apply a vacuum to the outside of the moldbefore the metal in the mold cavity has solidified, and

e. continuing to apply the vacuum to the outside of the mold and tomaintain the increased pressure on the metal through the mold openingwhile the metal at least partially solidifies.

11. A method as claimed in claim 10 including the step of heating themold so that it is hot during introduction of the molten metal.

12. In the casting of a gas permeable mold having an opening into whichmolten metal is introduced to fill a cavity of the mold, a methodcomprising the steps of:

a. mounting the mold so that its opening is in communication with avacuum casting chamber, evacuating the casting chamber and thenintroducing molten metal into the mold opening to fill the cavity,

c. thereafter increasing the pressure on the metal in the mold throughthe mold opening,

providing a sealed chamber around the mold,

e. evacuating the sealed chamber to apply a vacuum on the outside of themold before the metal in the mold cavity has solidified, and] f.continuing to apply the vacuum to the outside of the mold and tomaintain the increased pressure on the metal through the mold openingwhile the metal at least partially solidifies.

1. In the casting of a gas permeable mold having an opening into whichmolten metal is poured to fill a cavity of the mold, a method comprisingthe steps of: a. evacuating the cavity of the mold and introducingmolten metal while under a vacuum into the mold opening to fill thecavity, b. thereafter increasing the pressure on the molten metalthrough the mold opening, and c. Applying a vacuum to the outside of themold prior to solidification of the metal.
 2. In the casting of a gaspermeable mold having an opening into which molten metal is introducedto fill a cavity of the mold, a method comprising the steps of: a.applying a vacuum to the opening of the mold, b. introducing moltenmetal under a vacuum into the mold opening to fill the cavity, c.thereafter relieving the vacuum applied to the mold opening so that thepressure on the molten metal in the mold is increased, and, d. applyinga vacuum to the outside of the mold prior to solidification of themetal.
 3. In the casting of a gas permeable mold having an opening intowhich molten metal is introduced to fill a cavity of the mold, a methodcomprising the steps of: a. removing air form the inside of the moldthrough the mold opening, b. introducing molten metal under a vacuuminto the mold opening to fill the cavity, c. thereafter exposing theopening of the mold to atmospheric pressure while the metal is stillmolten, d. applying a vacuum to the outside of the mold prior tosolidification of the metal, and e. maintaining the mold opening exposedto atmospheric pressure and continuing to apply a vacuum to the outsideof the mold as the metal at least partially solidifies.
 4. A method asclaimed in claim 3 wherein the vacuum is applied to the outside of themold prior to the step of introducing molten metal into the moldopening.
 5. In the casting of a gas permeable mold having an openinginto which molten metal is introduced to fill a cavity in the mold, amethod comprising the steps of: a. mounting the mold So that its openingis in communication with a vacuum casting chamber, b. evacuating thechamber and then introducing molten metal into the mold opening to fillthe cavity, c. increasing the pressure in the chamber while the metal inthe mold is still molten, d. applying a vacuum to the outside of themold prior to solidification of the metal, and, e. continuing theapplication of the vacuum as the metal at least partially solidifies. 6.A method as claimed in claim 5 wherein the vacuum is applied to theoutside of the mold before introducing the molten metal into the moldopening.
 7. In the casting of a gas permeable mold having an openinginto which molten metal is introduced to fill a cavity of the mold, amethod comprising the steps of: a. mounting the mold in a mold chamberwith the opening of the mold communicating with a vacuum casting chamberwhich is sealed from the mold chamber, b. evacuating the vacuum castingchamber and then introducing molten metal into the mold opening to fillthe cavity, c. thereafter restoring pressure to the vacuum castingchamber while the metal in the mold is still molten, and d. evacuatingthe mold chamber prior to solidification of the metal.
 8. A method asclaimed in claim 7 wherein the mold chamber is evacuated before the stepof introducing the metal into the mold opening.
 9. A method as claimedin claim 7 including the step of preheating the mold before placing itin the mold chamber.
 10. In casting a gas permeable mold having anopening into which molten metal is introduced to fill a cavity of themold, a method comprising the steps of: a. evacuating the cavity of themold and introducing molten metal under vacuum into the mold opening tofill the cavity, b. thereafter causing the pressure on the molten metalwhich is exposed through the opening of the mold to be increased, c.providing a sealed chamber around the mold, d. evacuating the chamber toapply a vacuum to the outside of the mold before the metal in the moldcavity has solidified, and e. continuing to apply the vacuum to theoutside of the mold and to maintain the increased pressure on the metalthrough the mold opening while the metal at least partially solidifies.11. A method as claimed in claim 10 including the step of heating themold so that it is hot during introduction of the molten metal.
 12. Inthe casting of a gas permeable mold having an opening into which moltenmetal is introduced to fill a cavity of the mold, a method comprisingthe steps of: a. mounting the mold so that its opening is incommunication with a vacuum casting chamber, b. evacuating the castingchamber and then introducing molten metal into the mold opening to fillthe cavity, c. thereafter increasing the pressure on the metal in themold through the mold opening, d. providing a sealed chamber around themold, e. evacuating the sealed chamber to apply a vacuum on the outsideof the mold before the metal in the mold cavity has solidified, and f.continuing to apply the vacuum to the outside of the mold and tomaintain the increased pressure on the metal through the mold openingwhile the metal at least partially solidifies.